1. Field of the Invention
The present invention relates to a semiconductive ceramic, more specifically to a semiconductive ceramic having negative resistance-temperature characteristics. The present invention also relates to a semiconductive ceramic element making use of the semiconductive ceramic.
2. Background Art
Semiconductive ceramic elements having high resistance at ambient temperature and negative resistance-temperature ("NTC") characteristics by which resistance decreases with increase of temperature (such an element having NTC characteristics hereinafter called an "NTC element") are known. Through utilization of the NTC characteristics, the NTC element has been used for various purposes such as the suppression of rush current, retarding starting of a motor, and protection of halogen lamps.
For example, a NTC element rush current device suppresses overcurrent by absorbing the initial rush current, so as to prevent destruction of a halogen lamp or a semiconductive element such as an IC or diode due to overcurrent flowing through a circuit upon the switching-on of the power, and to prevent a decrease in service life of such a component. Thereafter, the NTC element reaches high temperature through self-heating so that its resistance decreases and, in a steady state, power consumption is reduced accordingly.
When current is passed through a motor for a gear mechanism--the motor being designed to be fed with a lubricating oil after starting of the motor--so as to rotate the gears immediately at a high rotational speed, the gears may be damaged due to insufficient supply of the lubricating oil. Also, in a lapping machine which grinds the surface of a ceramic through rotation of a grinding stone, the ceramic may be cracked when the lapping machine is rotated at a high rotational speed upon starting of the drive motor. In order to avoid these problems, the terminal voltage of the motor is lowered by means of an NTC element, so as to retard the starting of the motor. Thereafter, the NTC element comes to have lowered resistance through self-heating, so that the motor runs normally in a steady state.
As a semiconductive ceramic having NTC characteristics and constituting these NTC elements, there has been used spinel composite oxides containing a transition metal element such as Mn, Co, Ni or Cu.
Lanthanum cobalt oxides have been reported to have NTC characteristics such that the B constant has temperature dependency; that is, the B constant increases with increase of temperature (V. G. Bhide and D. S. Rajoria at al. Phys. Rev. B6, [3], 1072, 1972, etc.).
When an NTC element is used for suppression of rush current, it must have a decreased resistance at the elevated temperature caused by self-heating. However, the conventional semiconductive ceramic using a spinel composite oxide usually has a tendency that the B constant decreases as resistance decreases. Therefore, at an elevated temperature the resistance cannot be sufficiently lowered, with the result that power consumption is not reduced in a steady state.
Also, in a conventional semiconductive ceramic, resistance increases considerably at low temperatures below 0.degree. C., resulting in a voltage drop which retards the start-up of an apparatus or machine.
A conventional semiconductive ceramic element making use of conventional lanthanum cobalt oxide has a B constant of as high as 6000 K at an elevated temperature. However, since it has a B constant of 4000 K or more at low temperature, when used as an NTC element for suppression of rush current, the apparatus or machine to which the element is incorporated suffers a considerable voltage drop at low temperature.